Field Effect Transistors (FETs) are semiconductor transistor devices in which a voltage applied to an electrically insulated gate controls flow of current between source and drain. One example of a FET is a metal oxide semiconductor FET (MOSFET), in which a gate electrode is isolated from a semiconducting body region by an oxide insulator. When a voltage is applied to the gate, the resulting electric field generated penetrates through the oxide and creates an “inversion layer” or “channel” at the semiconductor-insulator interface. The inversion layer provides a channel through which current can pass. Varying the gate voltage modulates the conductivity of this layer and thereby controls the current flow between drain and source. MOSFETs may have different structures. In one example, MOSFETs may have a planar structure having gate, source and drain on the top of the device, with current flow taking place in a path parallel to the surface. In another example, MOSFETS may have a vertical structure in which a trench filled with doped polysilicon extends from the source to the drain with sidewalls and a floor that are each lined with a layer of thermally grown silicon dioxide. Such a trench MOSFET transistor allows less constricted current flow and, consequently, provides lower values of specific on-resistance.
FETs are useful in many power switching applications. In one particular configuration useful in a battery protection circuit module (PCM), two FETs are arranged in a back-to-back configuration with their drains connected together in a floating configuration. FIG. 1A schematically illustrates such a configuration. FIG. 1B shows use of such a device 100 in conjunction with a Battery Protection Circuit Module PCM 102, battery 104, and a load or charger 106. In this example, the gates of the charge and discharge FETs 120 and 130, respectively, are driven independently by a controller integrated circuit (IC) 110. This configuration allows for current control in both directions: charger to battery and battery to load. In normal charge and discharge operation both MOSFETs 120 and 130 are ON (i.e., conducting). During an overcharge or charge over-current condition of the battery 104, the controller IC 110 turns the charge FET 120 off and the discharge FET 130 on. During an over-discharge or discharge over-current condition, the controller IC 110 turns the charge FET 120 on and the discharge FET 130 off.
It is within this context that embodiments of the present invention arise.